A common type of pumping unit used in oil well applications includes a downhole pump located in an underground reservoir, and connected to the surface by a string of sucker rods. A drive apparatus on the surface is connected to the top of the rod string and moves the rod string up and down in a reciprocating manner to drive the pump and pump fluid to the surface.
One common type of reciprocating drive for use with such pumping units is disclosed for example in U.S. Pat. No. 4,949,784 to Evans. The drive comprises a motor driving an eccentric arm to cause a pump am to move up and down, and the top end of the rod string is connected to the end of the pump arm.
It is also well known to use a hydraulic cylinder to cyclically extend and retract to reciprocate the rod string. U.S. Pat. Nos. 4,512,149 to Weaver and 4,761,120 to Mayer et al. disclose such a reciprocating drives where the hydraulic cylinders are oriented vertically, while U.S. Pat. No. 4,530,645 to Whatley et al. discloses such a reciprocating drives where the hydraulic cylinder is oriented horizontally.
A consideration when designing a reciprocating drive for such oil well pumping units is the need to move the drive apparatus away from the top of the well to provide access for service rigs to be set up over the well head to provide periodic maintenance such as is usually required on oil wells. Such maintenance often requires that a service rig be positioned to pull the rod string and the downhole pump out of the well. Considerable time and equipment is typically required to disassemble and/or move existing prior art drives sufficiently to allow the service rig to be set up in the required location.